Nonlinear optical crystals are used extensively throughout the world in a great many optical applications. But for problems associated with radiation damage, their use would be even greater. Currently, the most common use of nonlinear crystals appears to be as second harmonic generators (SHGs) and as optical parametric oscillators (OPOs).
Most of the nonlinear crystals now being produced are capable of handling relatively high power radiation over a limited period of time. The useful lifetime of these nonlinear crystals is limited because the large electric fields associated with most crystal applications cause, over time, electrochromic and photochromic damage to the crystal. This progressive damage can severely limit the average-power use of these expensive crystals.
Single crystal KTiOPO.sub.4 (KTP) probably is the most widely used nonlinear optical material currently available. The excellent crystal stability and large nonlinear optical coefficient make KTP crystals the material of choice for both SHG and OPO applications. Because of the extensive use of KTP crystals, the radiation damage suffered by most nonlinear crystals is documented extensively for KTP crystals.
In the approximately twenty years which have elapsed since the commercialization of KTP crystals, a great deal of work has been devoted to understanding the origin of this radiation damage and developing effective methods for mitigating its effects. This radiation damage, also referred to as "photochromic damage" or "gray tracking," previously has been thought to occur when an oxygen-hole pair is generated and the electron is trapped on a Ti.sup.4+ ion, creating a stable Ti.sup.3+ ion. Optical absorption of Ti.sup.3+ ions occurs in the green region of the spectrum, and has been thought to be responsible for the gray tracking problem.
In SHG applications, the converted green light (.about.530 nm) was considered to be absorbed by the Ti.sup.3+, leading to heating and eventual fracture of the KTP crystal. It was postulated that the above-gap energy necessary for the creation of an oxygen-hole/electron pair can result from either sum-frequency mixing of the fundamental and second harmonic frequencies, sum frequency mixing of the Raman-shifted fundamental and second harmonic frequencies, or direct two-photon absorption of the generated green light.
Electrochromic damage in nonlinear crystals generally occurs when exposed to electric fields of a few kV/cm for a period of time. After a threshold field has been reached, the crystal breaks down, and green/black streaks appear in the material along the direction of the applied field. These streaked areas in the crystals exhibit absorption similar to the gray-tracked material, suggesting the conclusion that, in KTP crystals, the Ti.sup.3+ ions were associated with the damage.
When damaged crystals were cut open, an internal examination revealed an excess of Ti and K on the crystal's surface, and a deficiency of these elements in the internal regions. This finding led to the conclusion that Ti and K migrate under the application of a sufficiently strong electric field, and that the electrochromic damage was most likely a combination of bulk damage caused by this migration and the concomitant creation of Ti.sup.3+ ions in the KTP crystal. The present invention teaches that this is not the damage mechanism.
It is therefore an object of the present invention to provide methods for minimizing or preventing electrochromic damage to nonlinear crystals.
It is another object of the present invention to provide apparatus for allowing the electrical grounding of crystals.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.